1. Field of the Invention
This invention relates to the preparation of semiconductor alloys of the formula Hg.sub.(1-x) Cd.sub.x Te.sub.1, generically referred to as "HgCdTe" where X is in the range of from about 0.14 to about 0.40 for infrared applications.
2. Description of the Prior Art
Alloys containing mercury, cadmium, and tellurium having the general formulation set forth hereinabove have been reported in the prior art, particularly as having semiconductor properties, and have been mentioned particularly in conjunction with their use as infrared detectors and the like. Some of the principal requirements of the materials used in such applications are (1) compositional uniformity, i.e., constant of the alloying parameter X throughout the alloy; (2) low carrier concentration due to unintentionally introduced impurities (ideally less than 10.sup.15 per centimeter.sup.3 ; (3) good crystal quality (low density of vacancies and of dislocations); and (4) homogeneity of the distribution of the dopant in the alloy.
In the prior art, the production of homogeneously doped HgCdTe has presented a problem because gettering of the impurities has been demonstrated to occur in this alloy such that the fast diffusing dopant impurities such as copper, silver, or gold and possibly others are swept into the core of a partially post-annealed alloy slice. This is a particular problem since these attractive P-type dopants are very mobile. The reasons for this situation appear to be as set forth hereinbelow.
In accordance with prior art, doped slabs of HgCdTe have been prepared by initially preparing an ingot thereof by placing the appropriate amounts of mercury, cadmium, tellurium, and the impurity into a quartz ampoule, sealing the ampoule and then heating the ampoule and contents to a temperature in excess of 800.degree. C. until a liquid has been formed and then retaining that temperature for about a half day. The liquid is then shaken to distribute the contents of the ampoule therein and the ampoule is then quenched by blowing nitrogen gas thereon to provide the solid ingot within the ampoule. The ingot provided is non-homogeneous and polycrystalline in nature, being formed of small crystals. These crystals have random crystallographic orientations. The ingot is then made to undergo recrystallization and homogenization which is essentially an annealing step which takes place for a period of about four weeks at a temperature in excess of 650.degree. C. but below the melting point of the crystal. During recrystallization, one or more large crystals are formed from the polycrystalline material. This process permits solid state diffusion of the components of the ingot to provide a much greater degree of homogeneity in the large crystal ingot. The ingot is then cut into slabs or slices of predetermined thickness and the slabs are then subjected to a post-annealing step in a saturated mercury vapor atmosphere for a period of one to four weeks at a temperature below 300.degree. C., preferably about 280.degree. C., (so as to reduce the metal vacancy concentration of the outer surface by in-diffusing Hg. This process leaves a device quality region known as the skin depth ranging from about 2 to 15 mils.) The post-anneal of thick slices greater than twice the skin depth leaves a core of second phase tellurium as well as impurities in the central portion of the slab. As can be seen, though there is a great degree of homogeneity at the skin or surface of the slab, the fast diffusing impurities, are gettered in the central region or core of the slab by vacancies associated with the Te precipitates there, and therefore do not provide the desired homogeneous distribution of impurity or doping agent throughout the slab as desired.